The Park chain tool checks a chain for wear for a chain that is shifted. Since you don't shift a sync chain, it can stand a lot more wear before it exhibits any wear symptoms. I think that we have over 13,000 miles on our old sync chain and it doesn't show any wear problems. If your sync chain get noisy or wants to skip, then it is probably time to change. However, if your sync chain shows problems, it is likely very worn and you would probably need to change your sync rings as well (they wear together).